A ferrite has a magnetoplumbite-type (or M-type) crystal structure in the composition of AO·nFe2O3 (wherein A is a divalent metal ion, Sr or Ba). Since a ferrite is a material whose magnetic properties are not easily changed by the direction and magnitude of a magnetic field, it is commonly used as a material for permanent magnets to be employed in automobile electric motors, rotors for electric devices, and so on. In the meantime, it has been recently demanded that motors become smaller in size and more efficient due to environmental problems and various laws and regulations related to energy saving in accordance therewith, and permanent magnets are also required to have higher performance.
An M-type ferrite is characterized by uniaxial magnetocrystalline anisotropy among the essential requirements for a magnet material and contains twelve (12) iron ions per molecule as represented by its chemical formula of SrFe12O19 or BaFe12O19. The magnetic properties of an M-type ferrite are based on the magnetic moments of Fe ions. There are eight (8) upward spin directions and four (4) downward spin directions among the twelve (12) Fe ions, and the sum of the net spins per molecule for an M-type ferrite is four (4). In general, the iron ions in an M-type ferrite are trivalent (Fe3+) and have five (5) 3d electrons (i.e., spin magnetic moments). Thus, the twenty (20) spin magnetic moments (i.e., 5 spin moments×sum of the net spins) serve as the source of magnetism (i.e., saturation magnetization) of an M-type ferrite magnet. That is, in the case where some of the iron ions are substituted with either a non-magnetic element or an element having a spin magnetic moment lower than that of the iron ions in the downward spin direction of the iron ions for the purpose of improving the saturation magnetization value, which is the source of magnetism, the total spin magnetic moments are increased, resulting in an improvement in the saturation magnetization. In general, examples of the element having a spin magnetic moment lower than that of an iron ion include Co2+ (spin moment=3), Ni2+ (spin moment=2), Cu2+ (spin moment=1), and so on. The non-magnetic element is, for example, Zn2+ (spin moment=0). It is well known that if iron ions (Fe3+) are substituted with Zn2+, the total spin magnetic moments are increased to the maximum in theory, thereby producing a high saturation magnetization. Although Zn is capable of producing a high saturation magnetization, however, it has a low anisotropic magnetic field. Therefore, since Zn has a low coercive force value, it is difficult for Zn to be employed in permanent magnets.
In addition, in order to improve the magnetocrystalline anisotropy, which is one of the crucial magnetic properties of a permanent magnet, some of the iron ions in an M-type ferrite are substituted with an element having a high magnetocrystalline anisotropy, thereby producing a high anisotropic magnetic field.
The representative magnetic properties of a permanent magnet include residual magnetic flux density (Br), intrinsic coercive force (iHc), maximum energy product ((BH)max), and squareness ratio (Hknie/iHc). The intrinsic coercive force (iHc) and the residual magnetic flux density (Br) meet the following relationships:Br=4πIs×ρ×f (Is: saturation magnetization; ρ: density; and f: degree of orientation)iHc=HA×fc (HA: anisotropic magnetic field; and fc: volume ratio of single magnetic domains).
The residual magnetic flux density (Br) is proportional to the saturation magnetization, which is the sum of the spin magnetic moments of a composition, the density, and the degree of orientation. The density and the degree of orientation are physical properties materialized after fine pulverization in the process for preparing a ferrite. The residual magnetic flux density can be attained up to about 95% of the theoretical value (about 4,500 G) by process optimization. The theoretical value of the saturation magnetization for strontium ferrite (hereinafter referred to as “Sr-ferrite”) at room temperature is known to be 74 emu/g (4πIs=4,760 G, when the density and the degree of orientation are 100%, respectively). The saturation magnetization is increased by an increase in the spin magnetic moments in a substituted ferrite composition.
The intrinsic coercive force (iHc) is proportional to the anisotropic magnetic field and the volume ratio of single magnetic domains. It is known that the theoretical value of the anisotropic magnetic field for a single magnetic domain of Sr-ferrite is 20,000 Oe and that the crystal size of a single magnetic domain is about 1 μm. A high coercive force value can be achieved by increasing the volume ratio of single magnetic domains by process optimization after fine pulverization in the process for preparing a ferrite. It is possible to achieve about 40% (7,700 Oe) of the theoretical value due to the internal demagnetizing field when an Sr-ferrite is in the size of a single magnetic domain, and a higher anisotropic magnetic field value can be achieved by substituting some of Fe ions with an element having a high magnetic anisotropy. However, since the anisotropic magnetic field and the saturation magnetization have a trade-off relation according to the equation HA=2K1/Is (2K1: anisotropy coefficient), it is theoretically impossible to increase the saturation magnetization and the anisotropic magnetic field at the same time by a simple substitution of an element.
In the meantime, the maximum energy product ((BH)max) is the product of the magnetization (B) provided by a magnet and the magnetic field (H) acting on the magnet at each operating point on BH curves, which stands for the energy accumulated inside the magnet. On each demagnetization curve, the point at which the product of B and H is a maximum represents the maximum energy product. In general, a permanent magnet having high Br, iHc, and squareness ratio values has a high maximum energy product ((BH)max). A motor applied the permanent magnet has a high output and a low demagnetization caused by an external magnetic field. As a result, the maximum energy product is a representative performance index of a permanent magnet.
For example, U.S. Pat. No. 5,846,449 (Patent Document 1) discloses that in the case where some of Fe is substituted with Zn and some of Sr is substituted with La, a ferrite magnet having an improved saturation magnetization is obtained as compared with the conventional compositions in which some of Fe is substituted with Co. However, the ferrite magnet in which some of Fe is substituted with Zn involves a problem that the maximum energy product is lowered to 5.14 MGOe due to an abrupt decrease in the an isotropic magnetic field.
In addition, Korean Patent No. 0910048 (Patent Document 2) discloses a technique of improving the residual magnetic flux density and the intrinsic coercive force by way of substituting some of Ca with a rare earth element such as La and substituting some of Fe with Co, thereby producing a maximum energy product of 42.0 kJ/m3 (or about 5.28 MGOe). However, there is a problem that the magnetic properties of a magnet obtained in accordance with Patent Document 2 are not sufficiently high as compared with the conventional Sr-ferrite magnets (Patent Document 1).
Furthermore, Korean Patent No. 1082389 (Patent Document 3) discloses a method of obtaining high residual magnetic flux density, intrinsic coercive force, and squareness ratio values by way of substituting some of Ca with Sr, Ba, and La, and substituting some of Fe with Co and Cr. However, the maximum energy product of a magnet obtained by this method is 5.29 MGOe, which is not sufficiently high as compared with the conventional Sr-ferrite magnets (Patent Document 1). In addition, this method has a disadvantage in that a complicated sintering process is employed in order to obtain high magnetic properties, resulting in an increase in the manufacturing cost.
As described above, the ferrite magnetic materials of known compositions still have unsatisfactory magnetic properties. Accordingly, there has been a continued demand for a magnetic material that has excellent magnetic properties as compared with the conventional magnetic materials, to thereby meet the recent requirements such as high performance, high efficiency, miniaturization, and light weight of rotors and sensors used in automobiles, electric devices, and home appliances.